I. Field of the Invention
The present invention relates generally to the treatment of wastewater, and more particularly to the reduction of undesirable gases in wastewater traveling through sewage collection piping systems.
II. Description of the Problem
In the treatment of wastewater, lift stations and pretreatment facilities are often plagued by two distinct, but related, problems: the generation and release of malodorous gases, caused by hydrogen sulfide and mercaptans, and structural damage caused by the same microbes responsible for the presence of the gases. Because the problem of structural damage to these facilities has a more direct economic impact upon the municipalities whose charge it is to treat the wastewater, this invention primarily involves methods for protecting wastewater lift stations and pretreatment plants from such damage. The structural damage is largely the result of biological attack with the microbes feeding off the gases released from the sewage as it passes through the structures. As part of this feeding, the microbes attack the substrate material, e.g. the walls, ceilings, and other surfaces, within the structure to which they are attached as an additional source of food. These mechanisms are discussed in detail in my copending application Ser. No. 08/492,200, as well as in my prior patent, U.S. Pat. No. 5,433,854, the disclosures of which are incorporated herein by reference. Areas where the climate is mild most of the year, such as Florida, experience such problems in an ongoing manner. Publications such as the Florida Water Resources Journal recount the problems in articles such as "Hydrogen Sulfide Corrosion Control", Odom, et al., July 1993, pp. 40-42. This article defines the problems and suggests the use of protective coatings within the piping and structures to "increase the expected life span" by protecting against attack. Importantly, the authors note that there is no solution in preventing the attack, only protecting against it.
The aforementioned patent and application concern the introduction of controlled amounts of ozone-containing air to the vapor space immediately above the liquid level within the lift stations and pretreatment structures. The quantity and method of introduction of the ozone-containing air is such that sufficient amounts of ozone are available at the surface of the liquid to destroy the gases which the microbes use as food as they evolve. Furthermore, the amount of ozone is such that its biocidal effects destroy the microbial mass on the interior surfaces of the structures, thereby preventing damage to the structure. Destroying the gases and bacteria result not only in the prevention of structural damage, but also in the elimination of foul odors which are ordinarily present.
In the course of applying the ozone-containing air to various structures, it became apparent that the amount of ozone required to accomplish the destruction of gases and biomass could be reduced if the quantity of gas being released from the water could be reduced. Furthermore, not only would a reduction in the quantity of the gas be a benefit, but reducing the concentration of certain odorous compounds, such as hydrogen sulfide and mercaptans, would also be advantageous. This is especially true in piping systems where the wastewater is pumped long distances in what are commonly referred to as "forced mains". These pipes often cover distances of several miles and are completely filled with wastewater. The long distances mean that the wastewater often stays in the piping system for many hours (over one day in some cases), which results in a total depletion of oxygen within the piping system. The lack of oxygen is often referred to as a "septic" condition, and it is generally accompanied by the production of very foul odors and large quantities of hydrogen sulfide gas.
In order to completely understand this phenomenon, one must envision the interior surface of the wastewater piping system. Since the wastewater is filled with microbes, the microbes form a film of biomass on the interior surface of the pipe. This is particularly true of large diameter piping where low flow velocities prevent the force of the flow from scouring the surface with solid particles of sand and other matter. This film of biomass adapts readily to its environment, and it evolves with the dominant species of microbes that survive best under those conditions.
In nearly all instances, microbes that produce hydrogen sulfide gas as a by-product of their consumption of sulfate-containing material (hereinafter referred to as "sulfur reducing bacteria" or "SRB's") will be the dominant species. These microbes do not require oxygen, are always present in human waste, and grow rapidly on the walls of the piping system. They also grow on the suspended solids in the waste and on that portion of the suspended solids that settle out in the pipe when there is little or no flow. This occurs because anaerobes, i.e. microbes that exist without oxygen, such as sulfur reducers, only reproduce when they are attached to a surface. This surface may be the wall of the pipe or some bit of suspended matter. When the wastewater is moving slowly or not at all, the hydrogen sulfide gas produced by these bacteria accumulates in the wastewater in their vicinity.
In addition to the SRB's, other anaerobic bacteria exist in the system. These may vary in nature, but the most prevalent bacteria which compete with the sulfur reducers are methanogens. These bacteria primarily produce methane gas rather than hydrogen sulfide, and they compete, along with other bacteria, for food and space within the system. Unfortunately, without intervention, the SRB's usually dominate most of the time.
Another aspect of these closed piping systems that is often overlooked is the fate of the methane gas and its role in the piping system. Methane is virtually insoluble in water, so it collects as a separate gas phase within the piping system. Piping designers have positioned so-called "air release valves" at strategic points within the piping to relieve what they mistakenly believed was air. In actual practice these valves are almost always out of service, because the biomass fouls the mechanism, causing them to fail in the open position. This failure often results in wastewater being discharged on the ground. Moreover, operators who must operate these valves manually are understandably reluctant to make frequent adjustments to the valves, because the concentration of hydrogen sulfide gas is very high and offensive to the area.
When the gas cannot be released through the valves, the insoluble methane gas collects at the top of the pipes until the bubble is large enough to move downstream. This condition contributes greatly to the problems in two key respects. First, the methane gas bubble interferes with the normal hydraulic flow within the piping. Second, the hydrogen sulfide gas is highly soluble in methane, and it can accumulate in the gas phase in concentrations many times that of the water phase concentration (as has previously been shown in my '854 patent). When this accumulation occurs and the bubble of high strength methane gas enters a lift station, very high concentrations of hydrogen sulfide gas are released, because the hydrogen sulfide is already in the insoluble methane gas phase. In such instances, depending upon a number of variables, the volume and concentration may be such that the methods described in my previous patent will be less effective, because there may not be enough available ozone within the vapor space. When this happens, noxious odors may still be a problem due to the hydrogen sulfide being vented. Clearly, this is an undesirable situation.
Therefore, the present invention is directed toward preventing the formation of large quantities of hydrogen sulfide and methane gas within the wastewater collection piping system prior to lift stations and pretreatment facilities. By preventing such formation, the amount of hydrogen sulfide and methane gas entering the facilities is kept within ranges wherein various ozone-treatment methods, such as that disclosed in the '854 patent, can be practiced without periodic disruptions. Furthermore, it allows those methods to be applied in places that would, under other circumstances, be impractical.